1. Field
One or more aspects of embodiments according to the present invention relate to chirp generators, and more specifically to a direct digital synthesis (DDS) chirp generator capable of responding to commands ds to change the slope, frequency or phase during operation.
2. Description of Related Art
Signal generators employing direct digital synthesis (DDS) may, in some applications, be composed of a digital circuit connected to a digital to analog converter (DAC). The digital circuitry of the digital to analog converter may operate at a clock rate that is a multiple, N, say, of the clock rate at which the digital circuit operates, and in this case the digital circuit may be required to generate N outputs per clock cycle which are then multiplexed into the DAC.
In the case of a chirp generator, i.e. a DDS system which generates a sinusoidal signal with a frequency that changes at a certain, e.g., piecewise constant, rate, the DAC may be driven from a set of sine tables, which in turn are indexed by phase produced by a double accumulator taking as input the chirp rate, which may also be referred to as the slope, i.e., the rate of change of frequency. In this configuration, the output of the first accumulator of the slope is the frequency, and the output of the second accumulator is the phase. When N samples are needed to be generated during each clock cycle, parallel versions of the double accumulator are employed where there is a separate double accumulator for each of the N samples constructed in a cycle. For this case, each double accumulator generates frequency and phase samples separated by N samples.
It may be advantageous for a chirp generator to have the capability to respond to commands to change the slope, frequency or phase during operation. With a conventional double accumulator, where the clock rate of the accumulator is the same as the DAC rate, responding to the command changes is easily implemented. Implementing this capability with parallel versions of the double accumulator, however, results in the need for a complex compensation scheme to maintain the desired analog output as commanded changes in slope, frequency, or phase take effect. Thus, there is a need for a simple system for generating a chirp by direct digital synthesis that is capable of responding to commands to change the slope, frequency or phase during operation and of generating N outputs per clock cycle.